1. Field of the Invention
The present invention relates to an apparatus for storing electrical energy in a superconductor. It has a general application in the storage of energy, namely in the laboratory, in industry, on board vehicles, in artificial satellites, orbiting stations, space probes, etc.
2. Description of Related Art
As from the earliest superconductors, the dream was to store in such materials energy brought into an electrical form by producing a current in a superconductor coil. The energy E stored in a coil of radius R placed in vacuum and traversed by a current I is: EQU E=1/4.mu..sub.O I.sup.2 ( 1)
in which .mu..sub.O is the magnetic permeability of the vacuum.
However, only a limited amount of energy can be stored under such conditions. Thus, with a current of 10.sup.5 Acm.sup.-2 and a coil of radius 0.5 m, a stored energy of 1570 Joules or 0.43 Wh is obtained. Naturally by increasing the radius of the coil, there is a proportional increase in the stored energy and with a radius 50 m coil 43 Wh would be obtained. However, such an energy level is still modest.
If it is also borne in mind that such a coil with a diameter of 100 m has to be immersed in liquid helium, it is apparent that this storage process is theoretical rather than practical.
The arrival of new oxide-based superconductors operating at temperatures of approximately 90K has made such installations somewhat less heavy, mainly through the replacement of liquid helium by liquid nitrogen. The latter is, however much less easy to use and has a much higher latent heat of vaporization (198 instead of 20 J/g). However, the energy quantities stored are still small.
Consideration could be given to the associatition with the storage ring of a high magnetic permeability material in order to proportionally increase the stored energy (relation 1). Thus, with a permeability equal to 10.sup.5, which corresponds to permalloy, it would be expected that there would be a stored energy equal to 43 kWh for a 0.5 m radius coil traversed by a current of 10.sup.5 Acm.sup.-2.
Unfortunately, a secondary phenomenon makes such a solution illusory. The magnetic induction produced by the coil would become too great, so that the material used would lose its super conducting property. Thus, with a field of 10.sup.5 A/m, the induction in vacuum is 0.125 Tesla. With permalloy, this induction would increase to 12.5.multidot.10.sup.3 Tesla, which is incompatible with the critical induction of the known superconductor materials, which is a few Teslas. However, even if advances in this respect were to be expected, so that in the near future critical inductions of about 10 Teslas could be awaited, the recommended solution is still inappropriate.